The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device suitable for guest-host type color display.
A guest-host type liquid crystal display device in which a dichroic dye is added to a liquid crystal material, is inferior in contrast to a twisted-nematic liquid crystal display device.
In order to overcome this problem, a method has been proposed in which a quarter-wave plate (namely, a .lambda./4-plate) is disposed as shown in FIG. 1 (for example, reference is made to Japanese Patent Application Laidopen Nos. 39664/1979 and 26756/1979, and others).
In FIG. 1, reference numerals 1 to 6 designate lights, 8 a liquid crystal layer in which liquid crystal molecules 9 and dichroic dye molecules 10 are aligned parallel to an X-direction, 7a and 7b a pair of substrates whose facing surfaces are provided with electrodes (not shown), 11 a quarter-wave plate having a thickness of 1 to 10 mm and made of quartz or transparent muscovite, and 12 a reflecting plate.
Referring to FIG. 1, when natural light 1 having a plurality of electric field components a impinges on the liquid crystal layer 8 through the substrate 7a, a component of the natural light parallel to the direction of the absorption axis of each dichroic dye molecule 10 (in this case, the X-component of the natural light) is absorbed by the dye molecules, and therefore light 2 having passed through the substrate 7b is linearly polarized light b having only the Y-component of the natural light. The light 2 is incident on the quarter-wave plate 11, and light 3 having passed through the quarter-wave plate 11 becomes left-circularly polarized light c in which an electric field vector rotates counterclockwise. The light 3 is reflected back from the reflecting plate 12. At this time, the direction of rotation of the electric field vector is reversed, and therefore light 4 reflected from the plate 12 is right-circularly polarized light d in which the electric field vector rotates clockwise. The light 4 is incident upon the quarter-wave plate 11, and light 5 having passed through the plate 11 becomes linearly polarized light e parallel to the X-direction. In other words, the quarter-wave plate 11 and the reflector 12 rotate the plane of polarization through 90.degree.. When the light 5 is incident on the liquid crystal layer 8 through the substrate 7b, the electric field vector of the light 5 is parallel to the direction of the absorption axis of each dichroic dye molecule. Thus, almost all of the X- and Y-components of the incident natural light are absorbed by the dye molecules, and therefore light 6 having passed through the substrate 7a is extremely low in intensity.
When a voltage is applied across the liquid crystal layer 8, the dichroic dye molecules 10 are inclined together with the liquid crystal molecules 9, to be oriented in a Z-direction. Thus, the direction of the absorption axis of each dye molecule 10 becomes parallel to the Z-direction. Accordingly, the incident light 1 is reflected back from the reflecting plate 12 and returns to the incident side, without being absorbed by the dye molecules 10.
The contrast ratio of a liquid crystal display device is expressed by a ratio of the intensity of the light 6 in the state where the voltage is applied across the liquid crystal layer 8 to the intensity of the light 6 in the state where no voltage is applied. By using such a quarter-wave plate as shown in FIG. 1, the intensity of the light 6 in the state where no voltage is applied, is made small, and therefore the contrast ratio is made high.
While the structure of a liquid crystal device of negative display type in which a liquid crystal layer becomes transparent upon the application of a voltage thereto, is shown in FIG. 1, the contrast ratio can also be improved by using such a quarter-wave plate in a liquid crystal device of positive display type too, that is, in a liquid crystal display device in which liquid crystal molecules 9 and dichroic dye molecules 10 are first oriented perpendicularly to substrates 7a and 7b, and the dye molecules are oriented parallel to the substrates upon the application of a voltage to a liquid crystal layer so that the liquid crystal layer exhibits a color.
In the case where the quarter-wave plate 11 is interposed between the substrate 7b and reflecting plate 12 as shown in FIG. 2, a distance l in the lateral direction between a reflecting point on the front surface of the quarter-wave plate 11 and a reflecting point on the reflecting plate 12 corresponding to the reflecting point on the front surface of the plate 11 becomes large due to the thickness of the quarter-wave plate, and therefore two images 13 and 14 are obtained when the device is viewed by an observer 100 in a direction making an angle .theta. with the normal to the substrate. Thus, the quality of display image is greatly deteriorated. Such a difficulty becomes remarkable as the viewing angle .theta. is larger, and therefore the viewing-direction characteristic of the device is not good. Further, the quarter-wave plate is expensive.